The role of cyclin C in cell cycle regulation is currently poorly characterized. Cyclin C was isolated as a gene which can rescue G1 cyclin nulls in yeast indicating it can complement G1 cyclin function. Cyclin C interacts with cdk8 and by homology with cdc8 this suggests it may regulate RNA polymerase activity. However, this function is inconsistent with the rescue results which indicate a more direct role of cyclin C in cell cycle regulation. The cyclin C gene in humans maps to chromosome position in the 6q 21 region. Importantly, Dr. Lahti has shown that in 80% of childhood acute lymphoblastic leukemias cyclin C is deleted suggesting a causal relationship between cyclin C deletion and transformation in these tumors.